The cast-iron trough that carries the Llangollen Canal over the Dee valley holds close to 1.5 million litres of water, and its plate joints were sealed with Welsh flannel and lead boiled in sugar. Two centuries later, they still hold. Pontcysyllte Aqueduct runs 307 metres between Trevor and Froncysyllte on eighteen hollow masonry piers, lifting a navigable waterway 38 metres above the River Dee. The Llangollen Aqueduct Canoe Tour Adventure tours that set out from Trevor Basin trace that exact line, bank to open air. Thomas Telford and William Jessop completed the structure in 1805, after ten years of work. Their solution was unusual. Rather than build a masonry channel heavy enough to hold water, they cast the trough in iron plates and let the piers taper towards the top to save mass. The towpath sits on brackets above the water along the eastern side; the western edge carries no railing at all, only the rim of the trough and the drop beyond it. Narrowboats and canoes share the same 3.4-metre channel, with centimetres to spare on either beam. The design has required no fundamental alteration since. UNESCO inscribed the aqueduct and eleven miles of the surrounding canal as a World Heritage Site in 2009, describing it as a masterpiece of the industrial age. The designation reaches west to the Horseshoe Falls weir at Llantysilio, where the Dee is tapped to feed the channel, and east through the Chirk Tunnel. No cargo has moved along this water in living memory. What moves now is leisure traffic: hire boats, towpath walkers, cyclists, and paddlers on the Pontcysyllte Aqueduct canoe crossing, who meet the structure at the only level from which its thinness is obvious. Locally the structure is the Traphont Ddŵr Pontcysyllte, and the valley beneath it is farmland and hanging woodland, little altered in outline since the surveys of the 1790s. Sheep graze the slopes below the piers. Trains cross the Dee on a later viaduct downstream, a reminder of the technology that ended the canal age within four decades of the trough being filled. A Llangollen Aqueduct Canoe Tour Adventure tour therefore reads twice over: as engineering, precise and economical, and as landscape, an iron line ruled across a green Welsh valley that has never stopped doing the work it was built for.